The invention relates to a method for producing a cold-rolled sheet or strip of superior strength having good formability especially stretch-formability for making pressings with a high buckling resistance.
The pressings are to be of high basic material strength and after additional heat treatment as it is usually applied for enamelling, they are to receive additional bake hardening. In this way, outstanding buckling resistance characteristics are achieved. For example body sheets in the motor vehicle industry, such as doors, hoods, roofs, are pressings comprising a high degree of stretch-forming.
In the production of continuous-annealed aluminium killed non-alloyed deep-drawing steels and which have particular requirements in respect of formability, after cooling from the recrystallization temperature, an additional annealing, so-called overageing annealing, is applied to ensure ageing stability. A non-ageing material is characterized in that even after extended storage periods no significant changes occur in the material's properties and further processing free of stretcher strain and free of defects is possible. In a continuous furnace such treatment can take place in an in-line overageing section. In the case of strip which is produced in a common hot-coating plant, subsequent external annealing, usually in the coil, needs to be carried out. Aluminium-killed non-alloyed deep-drawn steels, also called low-carbon (LC) steels, have a carbon content ranging from 0.02 to 0.08%.
Above all in motor vehicle body building, for reasons of weight reduction, the use of the thinnest possible sheet is desired. To provide the required buckling resistance in spite of sheets of reduced thickness, higher strengths are required. Increasingly, bake-hardening steels are used for this purpose. Steels with bake-hardening properties are characterized by an additional increase in yield strength of the drawn component. Such an increase is achieved in that the material, apart from the work hardening occurring during pressing, is subjected to an additional increase in strength, the so-called bake hardening. The physical reason for this is a carbon-ageing occurring under controlled conditions. Bake-hardening steels and their intended applications also require adequate ageing stability for surfaces free from imperfections after pressing.
In continuous furnaces comprising an in-line overageing section, a non-alloyed LC steel can also be produced as a bake hardening steel, in that the chemical composition of the steel, the rate of cooling and the overageing condition are exactly matched to each other. This process is already used on a commercial scale. Optimization of the production conditions is for example described by Hayashida et al. (T. Hayashida, M. Oda, T. Yamada, Y. Matsukawa, J. Tanaka: "Development and applications of continuous-annealed low-carbon Al-killed BH steel sheets", Proc. of the Symp. on High-strength sheet steels for the automotive industry, Baltimore, Oct. 16-19, 1994, p. 135).
In other processes for producing non-ageing cold-rolled steels with bake hardening properties in continuous strip plants, low-carbon steels, so-called ultra low carbon (ULC) steels are used. A process based on a ULC steel for hot-coating plants, partially stabilized with titanium, is described by N. Mizui, A. Okamoto, T. Tanioku: "Recent development in bake-hardenable sheet steel for automotive body panels", International conference "Steel in automotive construction", Wurzburg 24.-26.9.1990). The carbon content is to be between 15 and 25 ppm. The titanium content is matched to the nitrogen and sulphur contents with 48/14 N&lt;Ti&lt;48 (N/14+S/32). The aim is a complete binding of the nitrogen in titanium nitrides, however a small quantity of carbon must remain soluble to ensure the bake-hardening effect takes place. Production in a vacuum degassing plant is necessary. This process has the advantage that overageing annealing can be omitted, thus making it suitable for hot-coating plants. With steels produced in this way, the bake-hardening parameters determined in tension specimens after 2% initial elongation (BH.sub.2 value) are approx. 40 N/mm.sup.2. The yield strength is approx. 200 N/mm.sup.2 ; the values for average vertical anisotropy (r value) are approx. 1.8.
According to W. Bleck, R. Bode, O. Maid, L. Meyer: "Metallurgical design of high-strength ULC steels", Proc. of the symp. on high-strength sheet steels for the automotive industry, Baltimore, Oct. 16-19, 1994), for representing such ULC steels partially stabilized with titanium, titanium contents are between 0.6 times and 3.4 times the nitrogen content. The sum of carbon and nitrogen contents should not exceed 50 ppm.
EP 0 620 288 A1 discloses a process for producing steel strip which is only cold-rolled or hot-coated in continuous strip plants, with this steel strip apart from ageing stability also comprising high bake-hardening characteristics and good deep-draw characteristics due to high r values. A ULC steel on its own or a ULC steel either with a titanium alloy or an niobium alloy is annealed above the Ac.sub.3 transformation temperature, i.e. in the austenitic range. In this process, the bake-hardening values attain 100 N/mm.sup.2. No overageing annealing is necessary. As this is a ULC steel, steel production must take place in a vacuum degassing plant. The high annealing temperatures necessary with this process create difficulties regarding strip flatness. Application of this process on a commercial scale is not known.
Bleck et al. (op. cit.) point out that the production of a non-ageing steel with good shaping characteristics based on non-alloyed LC steels, is not possible without overageing, in continuous strip plants. Since the cooling process in hot-coating plants in current use is limited due to the hot-dip galvanizing setup, in-line overageing annealing as mentioned above cannot take place. Consequently, with the known state of the art, the production of non-ageing steels with bake-hardening properties, in hot-coating plants, is exclusively limited to ULC steels. Thus the processes, applied so far or described in the literature, for producing in continuous strip plants, cold-rolled sheet with good formability and which comprises bake-hardening properties, either necessitate the additional annealing treatment as described above (if a soft non-alloyed Al-killed deep-drawn steel is used), with such a production not being possible in a hot-coating plant; or else they necessitate the use of ULC steels of very low carbon content, with these steels being more expensive to produce. The processes described above based on ULC steels mainly comprise steels with yield strength in the lower region up to 240 N/mm.sup.2. Due to the high average r values (&gt;1.5) they are used for pressings with a high degree of deep drawing.